SR850 Basics

SIGNAL INPUT AMPLIFIER and FILTERS

A lock-in can measure signals as small as a few nanovolts. A low noise signal amplifier is required to boost the signal to a level where the A/D con- verter can digitize the signal without degrading the signal to noise. The analog gain in the SR850 ranges from roughly 7 to 1000. As discussed pre- viously, higher gains do not improve signal to noise and are not necessary.

The overall gain (AC plus DC) is determined by the sensitivity. The distribution of the gain (AC versus DC) is set by the dynamic reserve.

Input noise

The input noise of the SR850 signal amplifier is about 5 nVrms/√Hz. What does this noise figure mean? Let's set up an experiment. If an amplifier has 5 nVrms/√Hz of input noise and a gain of 1000, then the output will have 5 µVrms/√Hz of noise. Suppose the amplifier output is low pass fil- tered with a single RC filter (6 dB/oct roll off) with a time constant of 100 ms. What will be the noise at the filter output?

Amplifier input noise and Johnson noise of resis- tors are Gaussian in nature. That is, the amount of noise is proportional to the square root of the bandwidth in which the noise is measured. A single stage RC filter has an equivalent noise bandwidth (ENBW) of 1/4T where T is the time constant (RxC). This means that Gaussian noise at the filter input is filtered with an effective band- width equal to the ENBW. In this example, the filter sees 5 µVrms/√Hz of noise at its input. It has an ENBW of 1/(4x100ms) or 2.5 Hz. The voltage

noise

at

the

filter

output

will

be

5 µVrms/√Hz x√2.5Hz

or

7.9µVrms.

For

Gaussian noise, the peak to peak noise is about 5 times the rms noise. Thus, the output will have about 40 µV pk-pk of noise.

Input noise for a lock-in works the same way. For sensitivities below about 5 µV full scale, the input noise will determine the output noise (at minimum reserve). The amount of noise at the output is determined by the ENBW of the low pass filter. The SR850 displays the ENBW in the Time Constant menu. The ENBW depends upon the time constant and filter roll off. For example, sup- pose the SR850 is set to 5 µV full scale with a 100 ms time constant and 6 dB/oct of filter roll off. The

lock-in will measure the input noise with an ENBW of 2.5 Hz. This translates to 7.9 nVrms at the input. At the output, this represents about 0.16% of full scale (7.9 nV/5 µV). The peak to peak noise will be about 0.8% of full scale.

All of this assumes that the signal input is being driven from a low impedance source. Remember resistors have Johnson noise equal to 0.13x√R nVrms/√Hz. Even a 50Ω resistor has almost 1 nVrms/√Hz of noise! A signal source impedance of 2kΩ will have a Johnson noise greater than the SR850's input noise. To deter- mine the overall noise of multiple noise sources, take the square root of the sum of the squares of the individual noise figures. For example, if a 2kΩ source impedance is used, the Johnson noise will

be 5.8 nVrms/√Hz. The overall noise at the SR850 input will be [52 + 5.82]1/2 or 7.7 nVrms/√Hz.

We'll talk more about noise sources later in this section.

At lower gains (sensitivities above 50 µV), there is not enough gain at high reserve to amplify the input noise to a level greater than the noise of the A/D converter. In these cases, the output noise is determined by the A/D noise. Fortunately, at these sensitivities, the DC gain is low and the noise at the output is negligible.

Notch filters

The SR850 has two notch filters in the signal amplifier chain. These are pre-tuned to the line fre- quency (50 or 60 Hz) and twice the line frequency (100 or 120 Hz). In circumstances where the larg- est noise signals are at the power line frequencies, these filters can be engaged to remove noise sig- nals at these frequencies. Removing the largest noise signals before the final gain stage can reduce the amount of dynamic reserve required to perform a measurement. To the extent that these filters reduce the required reserve to either 60 dB or the minimum reserve (whichever is higher), then some improvement might be gained. If the required reserve without these notch filters is below 60 dB or if the minimum reserve is suffi- cient, then these filters do not significantly improve the measurement.

Using either of these filters precludes making

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SRS Labs SR850 manual Signal Input Amplifier and Filters, Input noise, Notch filters
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SR850 specifications

The SRS Labs SR850 is a high-performance audio processor designed to enhance the listening experience across a variety of applications. With its advanced technologies, the SR850 delivers superior sound quality that is particularly noticeable in environments where audio clarity and fidelity are paramount. This device caters to audio professionals, audiophiles, and casual listeners who demand exceptional performance from their audio systems.

One of the standout features of the SR850 is its proprietary SRS 3D Audio technology. This innovative feature creates a more immersive soundstage, allowing users to experience audio as if they were in a live performance setting. By adding depth and dimension, it elevates the listening experience, making music, movies, and games far more engaging.

Additionally, the SR850 incorporates SRS dialog clarity technology, which enhances speech intelligibility in dialogues and vocals. This is particularly beneficial in film and television viewing, where dialogue can sometimes be drowned out by background music or sound effects. With dialog clarity, users can enjoy crystal-clear conversation tracks without adjusting volume levels continuously.

The device also supports various audio formats, ensuring compatibility with a broad range of media. It boasts inputs for both digital and analog sources, allowing connections to various devices such as TVs, laptops, gaming consoles, and more. The versatility of inputs makes the SR850 a convenient addition to any home entertainment system.

Another impressive characteristic of the SR850 is its compact and user-friendly design. The intuitive interface makes it easy for users to adjust settings to their preference, whether they are a novice or an audio expert. The device is equipped with programmable presets, enabling users to store their favorite sound profiles, making switching between settings quick and hassle-free.

Furthermore, the SR850 features advanced EQ capabilities, granting users the ability to tailor the audio output to their specific tastes. This personalized approach to sound adjustment enhances the overall experience, allowing users to get the most out of their audio content.

In summary, the SRS Labs SR850 is a sophisticated audio processor packed with features designed to significantly uplift sound quality and listening pleasure. With its cutting-edge technologies such as SRS 3D Audio and dialog clarity, an array of connectivity options, and customizable EQ settings, the SR850 stands out as an outstanding option for anyone looking to enhance their audio experience. Whether for professional use in sound engineering or personal enjoyment in home entertainment, the SR850 proves to be a valuable asset.
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